Iran's detention of U.S. journalist Jason Rezaian hits 500 days on Thursday, with The Washington Post saying that its Tehran correspondent is in "immediate danger" as his health deteriorates.

The newspaper said Wednesday that it had made a new appeal to the U.N. Working Group on Arbitrary Detention for help in securing the release of the 39-year-old reporter, who holds dual Iranian and U.S. citizenship. The newspaper said Rezaian suffers from high blood pressure and other ailments as "Iran's mistreatment of him has intensified."

Iran announced last month that Rezaian had been sentenced to an undisclosed prison term following his conviction on espionage and other charges in October. But no details of the verdict or sentence have been released.

Post editor Martin Baron called Rezaian's 500th day of detention the "grimmest" of milestones. It meant he'd been held 56 days longer than Tehran held U.S. Embassy workers following the 1979 Islamic revolution.

"Five hundred days robbed of his life, 500 days deprived of his family, 500 days denied any semblance of justice," Baron said in a statement.

On Thursday, Rezaian's brother, Ali, is planning to deliver a petition to Iran's U.N. mission with more than 500,000 signatures calling for Rezaian's "immediate and unconditional release."